1. Field of the Invention
The present invention relates to a surface acoustic wave device and a method for manufacturing the same. More particularly, the present invention relates to a surface acoustic wave device having an improved high-power durability. The surface acoustic wave device of the present invention can be used as a surface acoustic wave device (a resonator, an interstage filter, a duplexer, or the like) in mobile communication terminals such as automobile phones and portable phones.
2. Description of the Related Art
Recently, development of smaller and less heavy mobile communication terminals such as automobile phones and portable phones has been rapidly taking place. In accordance with this development, there is an increasing need for smaller and more efficient parts for the mobile communication terminals, giving rise to the need also for surface acoustic wave (SAW) devices (resonators, interstage filters, duplexers, and the like) that contribute to miniaturization of high frequency (RF) sections.
Among the above duplexers, the antenna duplexer is typically located at the front end section of the RF section and requires large high-power durability. Here, since conventional SAW devices do not have sufficient high-power durability, dielectric filters have been used. Having a large size, however, the dielectric filters have been an obstacle to miniaturization.
On the other hand, in accordance with the development of SAW devices for higher frequency, the electrodes constituting the SAW devices have been miniaturized, and their high-power durability has been a point of issue in applying the devices to, for example, semi-microwave band filters.
In order to improve the high-power durability of the SAW devices, the following techniques have been reported.
1. Japanese Laid-open Patent Application (Kokai) No. SHO 63(1988)-278343 discloses a technique for improving high-power durability of the SAW device by utilizing an Al alloy having Ge added thereto at 0.1 to 5 wt % as a material for a comb-like electrode formed on a piezoelectric wafer of the SAW device so as to suppress the electromigration due to the movement of Al atoms caused by high density currents, thereby reducing the generation of defects (voids) or projections (hillocks). PA1 2. Japanese Laid-open Patent Application (Kokai) No. HEI 2(1990)-274008 discloses a technique for improving high-power durability of the SAW device by utilizing an Al alloy having Pd added thereto at 0.1 to 0.3 wt % or an Al alloy having Pd with the crystal diameter of less than 0.05 .mu.m added thereto at 0.1 to 0.3 wt % as a material for a comb-like electrode formed on a piezoelectric wafer of the SAW device so as to suppress stress migration due to the movement of Al atoms caused by high density currents, thereby reducing the generation of voids or hillocks. PA1 3. Japanese Laid-open Patent Application (Kokai) No. HEI 7(1995)-221578 discloses a technique for improving high-power durability by forming a film having a tensile stress on the back side of the piezoelectric wafer and by forming a film having a compressive stress on the comb-like electrode to utilize these stresses. PA1 4. Japanese Laid-open Patent Application (Kokai) No. HEI 5(1993)-14118 and Japanese Laid-open Patent Application (Kokai) No. HEI 5(1993)-3417 disclose a technique for improving high-power durability by dividing a SAW device into a plurality of unit filters formed on a common piezoelectric wafer or on separate piezoelectric wafers so as to constitute a system of filters disposed in series by connecting the input and output terminals of these unit filters in series, or a system of filters disposed in parallel by connecting the input and output terminals of these unit filters in parallel, thereby equally dividing a high power input by the number of unit filters. PA1 5. Japanese Laid-open Patent Application (Kokai) No. HEI 6(1994)-29779 discloses a technique for improving high-power durability in a ladder-like SAW device in which the comb-like electrodes are interdigitally aligned and in which the first resonators having a predetermined resonance frequency are arranged in a parallel arm and the second resonators having a resonance frequency at least approximately equal to the antiresonance frequency of the first resonators are arranged in a series arm in a plurality of stages. Here, high-power durability is improved by setting the number of electrode finger pairs of the series resonator at the first stage to be greater than the number of electrode finger pairs of the series resonators at the other stages so as to reduce the electric currents flowing through each of the electrodes in the second resonator at the first stage, thereby suppressing the temperature increase. PA1 (1) Article "Thin Film Alloyed Aluminum and its Deposition for High Power Durable SAW Devices" on pages 7-12 of the Preliminary Drafts for 17th EM Symposium (Yuhara et al.) describes use of Ti as a metal to be added into Al alloy. By changing the electrode material to be Al--Ti alloy, the lifetime of the SAW device was increased to 10 times that of the Al--Cu alloy film. However, with such an increase, the high-power durability is still insufficient. PA1 (2) Journal of Electronic Information Communication Society, Vol. J76-A, No. 2, pp. 145-152 (1993) (Ieki et al.) describes use of epitaxially grown Al single crystal film. This article utilizes a property that the use of a single crystal film can suppress intergranular diffusion in stress migration. PA1 (3) The stress migration in the SAW device is, on some points, similar to the electromigration or stress migration in the wiring technique of semiconductor devices. Accordingly, this wiring technique can be a reference for the anti-stress-migration technique in the SAW devices. An example thereof is a technique disclosed in U.S. Pat. No. 4,017,890 (April 1977, J. K. Howard in IBM) and reported, in association therewith, in J. K. Howard: J. Appl. Phys., Vol. 49, P. 4083 (1978).
However, even the above techniques do not provide sufficient high-power durability. Accordingly, the following techniques have been reported considering the material to be used for the electrodes constituting the SAW devices.
The increase of the lifetime by this method is reported to be 2000 times that of the Al--Cu alloy film formed by vapor deposition. Since the Al--Cu alloy film formed by vapor deposition naturally has a lifetime shorter by one to two orders of magnitude than the film formed by the sputtering method (See the article in the above (1)), it is understood that the lifetime is shorter substantially by 20 to 200 times. However, this method is disadvantageous in that the applicable underlying substrate or its cut surface is limited and, moreover, has poor mass-productivity.
This technique involves forming a layered intermetallic compound of Al and a transition metal in the middle of the Al film so as to block the electromigration of Al atoms by the intermetallic compound. This article discloses that, if Cr is used as the transition metal, the maximum lifetime attained is 10 times that of the Al--Cu alloy film. However, when the inventors of the present invention applied this technique to the electrodes of a SAW device, it was not possible to obtain a sufficient effect.
On the other hand, the inventors of the present invention have filed an application, Japanese Laid-open Patent Application (Kokai) No. HEI 7(1995)-122961, disclosing a technique for improving the high-power durability of the SAW device by laminating a plurality of Al--Cu alloy films and Cu films to form an electrode and utilizing the fine crystal grains constituting these films and the blocking effect of the alloy film.
Here, the weight of the electrode constituting the SAW device greatly affects a property such as the frequency. Therefore, the weight of the electrode is limited to a certain value in accordance with the use of the SAW device. When the technique disclosed in Japanese Laid-open Patent Application No. HEI 7(1995)-122961 is studied from this view point, it is to be understood that, since the specific weight of Cu is 3.3 times larger than the specific weight of Al, Cu will shift the property 3.3 times more than Al even if the change in the film thickness is the same.
Alternatively, a method of reactive ion etching (RIE) for forming a fine pattern of electrodes is known. However, it is known that it is difficult to apply this method to Cu.